\(x^4+y^4=\left(x^2\right)^2+\left(y^2\right)^2=\left(x^2+y^2\right)^2-2x^2y^2\)
\(\left(x+y\right)\left(x^3-x^2y+xy^2-y^3\right)\)
\(x^4+y^4\)
\(=\left(x^2\right)^2+2x^2y^2+\left(y^2\right)^2-2x^2y^2\)
\(=\left(x^2+y^2\right)-2x^2y^2\)
\(=\left(x^2+y^2-\sqrt{2xy}\right)\left(x^2+y^2+\sqrt{2xy}\right)\)
(Phương pháp thêm bớt hạng tử)